Before certain gases can be used in an industrial process, they must firstly be stripped of the impurities that they contain.
Thus, atmospheric air, which contains approximately 250 ppm to 500 ppm of carbon dioxide (CO.sub.2) and a variable amount of water vapour and hydrocarbons, must be stripped of the CO.sub.2 and optionally H.sub.2 O and hydrocarbon impurities prior to any operation of cryogenically separating the air, especially by cryogenic distillation.
This is because, in the absence of such a pretreatment of the air, the CO.sub.2 and optionally H.sub.2 O and hydrocarbon impurities which are in it would solidify at low temperature, and the distillation columns would then become blocked, leading, on the one hand, to possible damage of the equipment and, on the other hand, to incorrect separation of the various constituents of air, such as nitrogen or oxygen.
Furthermore, the hydrocarbons likely to be present in the atmospheric air may, in some cases, build up in the liquid oxygen in the cold box and, for obvious safety reasons, it is then necessary to reduce their concentration in the liquid oxygen to as low a level as possible so as thus to prevent any damage of the plant.
A first technique for removing the CO.sub.2 and H.sub.2 O impurities contained in a gas stream, such as air, consists in refrigerating these impurities, that is to say in solidifying or crystallizing the impurities at low temperatures.
However, this technique is very costly in terms of both energy and equipment.
An alternative to this technique is to remove the carbon dioxide, and optionally the water, contained in the gas stream to be treated by adsorbing the impurities on a suitable adsorbent.
Zeolite-type materials are the adsorbents most commonly used in adsorption processes for gas separation.
Thus, document U.S. Pat. No. 3,885,927 describes the use of a zeolite-X at least 90% exchanged by barium cations, which zeolite has a CO.sub.2 adsorption capacity approximately 40% greater than a zeolite-X containing only sodium cations.
Furthermore, document EP-A-0,284,850 describes a process for purifying a gas stream by adsorption over a Faujasite-type zeolite whose Si/Al ratio ranges from 1 to 2.5, which Faujasite is at least 90% exchanged by divalent cations, such as strontium or barium cations.
Moreover, document U.S. Pat. No. 4,775,396 describes the preferential adsorption of carbon dioxide contained in a non-acid gas, such as nitrogen, hydrogen and methane, by a PSA (Pressure Swing Adsorption) process over a fixed adsorption bed containing a Faujasite-type zeolite at least 20% exchanged by cations of the group formed by zinc, rare earths, hydrogen and ammonium and at most 80% exchanged by alkali metal or alkaline-earth metal cations.
As regards document FR-2,335,258, this describes a process for purifying gas mixtures comprising nitrogen, hydrogen, argon and/or oxygen, and containing carbon monoxide, carbon dioxide and water impurities, by adsorption of the impurities over A-type or X-type zeolites at a temperature of between -40 and +4.degree. C.
That document describes A-type zeolites which are 70 to 82% exchanged by calcium ions and which have a Si/Al ratio of at most 1, and exchanged or unexchanged X-type zeolites which have a Si/Al ratio of between 1.15 and 1.5.
Conventionally, X-type zeolites whose Si/Al ratio is less than 1.15 are called LSX (Low Silica X-zeolites) or silica-depleted zeolites.
Likewise, document EP-A-0,718,024 describes the removal of CO.sub.2 contained in a gas stream by adsorption of the CO.sub.2 over a zeolite-X whose Si/Al ratio is at most approximately 1.15 at a temperature of between -50.degree. and +80.degree. C. approximately. That document furthermore describes results obtained by using X- or LSX-type zeolites, which are unexchanged or exchanged with lithium and calcium or rare-earth cations.
Moreover, mention may also be made of document EP-A-0,667,183 which describes the use of X-type zeolites containing from 50 to 95% of lithium cations, from 4 to 50% of aluminum, cerium, lanthanum or mixed-lanthanide cations, and from 0 to 15% of other cations.
Furthermore, documents GB-A-1,589,213, U.S. Pat. No. 5,258,058 and GB-A-1,551,348 also exist which describe processes for purifying gas mixtures by means of various exchanged or unexchanged zeolites.